The present invention relates to photolytically-induced decompositions such as water photoelectrolysis, particularly with a multielectrode semiconductor photoelectrochemical cell capable of unassisted photolytic water splitting to form H.sub.2 and O.sub.2 and methods of making and using same. The present invention does not require that the semiconductor have a flatband potential more negative than the reduction potential of H.sub.2 O or that the semiconductor be stable with respect to photo-oxidation while evolving oxygen.
Photolytically induced decompositions, particularly the photolysis of water into H.sub.2 and O.sub.2 using solar radiation, have received extensive attention and various photoelectrochemical devices and methods for this purpose have been developed. However, in many cases, an expenditure of electrical or chemical energy in addition to incident radiant (e.g., solar) energy is required. Because the potential developed by a single photoelectrode in these devices is inadequate to drive the water splitting reaction at a useful rate with visible light, an external bias has been necessarily applied. This bias has been either an external electrical potential or a chemical bias established by contacting semiconductor sides of bipolar photoelectrodes with a strong alkali and metallic sides with a strong acidic solution. Although it is known that connecting several photoactive junctions in series may generate sufficient driving force to decompose H.sub.2 O to H.sub.2 and O.sub.2 without an external bias, there are still some problems related to earlier devices such as complicated construction or difficulties in the separation and collection of H.sub.2 and O.sub.2, thus impeding practical application. Examples of attempted practical photoelectrochemical devices are seen in White et al., (J. Electrochem. Sol. Vol. 132, p 544 (1985)), and in U.S. Pat. No. 4,094,751, issued June 13, 1978.
Studies of water photoelectrolysis ("water splitting") with TiO.sub.2 and Pt electrodes date from the work of Honda et al., (Nature, (1972), v. 238, pp. 37). Because the potential developed by platinized TiO.sub.2 is inadequate to drive the water splitting reaction at a useful rate, an external bias must be applied. This bias can be either an external electrical potential or a chemical bias established by contacting the TiO.sub.2 with a strong alkaline and the Pt with a strong acidic solution. (Wrighton, Proc. Nat. Acad. Sci., (1975), U.S.A., V 72, pp. 1518).
In such cases an expenditure of energy in addition to the incident radiant (e.g., solar) energy is required. An alternative strategy involves the utilization of one or more PEC cells to bias the water splitting cell. Earlier work (White et al., cited above) showed, for example, that this could be accomplished by coupling of Texas Instruments solar energy system arrays based on Si p/n junctions. Simpler types of bipolar electrodes, and arrays based on these, have not previously been described.
A photoelectrochemical device which is of relatively simple construction, allowing ready collection of H.sub.2 and/or O.sub.2 and not requiring any external bias has not, prior to the invention described herein, been developed for photolytically-induced decompositions such as the photolysis of water into H.sub.2 and O.sub.2 using light from, for example, solar radiation.